The light-emitting mechanism and the structure of a light-emitting diode (LED) are different from that of the conventional light source. The LED owns the advantages of small size and high reliability, and is applicable diversely in the market. For example, LED can be produced as all kinds of large-sized devices for indoor or large outdoor displays in compliance with different needs.
Taking the blue LED chip of GaN series as example, because the sapphire substrate is an insulating substrate, the p type electrode and the n type electrode of the blue LED chip are located on the same side of the blue LED chip. When being encapsulated, the blue LED chip is mounted on a carrier in a manner of the electrodes of the blue LED chip facing upward and the sapphire substrate facing the carrier. The bond pads are formed on the p type and the n type electrodes respectively. The p type and the n type electrodes electrically connect to the carrier through golden wires in a wiring manner respectively. Finally, the blue LED chip is encapsulated by transparent encapsulating materials. Because it takes spaces to accommodate the golden wires the size of the encapsulated LED is larger, and makes it not applicable to the applications with size restriction like back light module.
To reduce the size of the LED, solder bumps are developed on the electrodes of the LED chip, and melted to become a golden ball after reflow. The chip is then reversely mounted on the carrier in a manner of the electrodes facing the carrier. The pads of the carrier are mounted with the two electrodes to form a flip chip structure with electrical connection. However, when the metal is melted to form the solder bump, it spreads to other areas of the chip or the carrier and makes the device short. Furthermore, the manufacturing process is complicated and the cost is high.
In addition, the metals with lower melting point, such as PdSn layer, can replace the solder bumps. With ultrasonically heating technology for bonding, the eutectic reaction is formed between the pads of the PdSn layer and the carrier to mount the chip and the carrier for electrical connection. However, this method needs better flatness of the surfaces of the bond pad and the carrier. If the surfaces are rough, the problems of chip peeling and insufficient mounting force usually occur after mounting.
Another bonding technology is using opaque anisotropic conductive film as an adhesive between the chip and the carrier. In general, the anisotropic conductive film has the conductive particles spread in the epoxy. When the chip is mounted on the carrier through heating and pressurization, the conductive particles can contact the chip and the carrier to generate electrical connection. Because the anisotropic conductive film is opaque, light emitting from the chip to the carrier is absorbed. Thus, a reflecting apparatus should be provided between the chip and the anisotropic conductive film to reflect the light emitted to the reflecting apparatus to the upper side of the chip. However, because of the internal total reflection, a portion of the light is reflected to inside of the LED and the possibility of absorption by the active layer is increased. Consequently, the brightness and the efficiency are reduced.
Therefore, it is necessary to provide a light-emitting device with stronger mounting force between the chip and the carrier and improving brightness and the light-emitting efficiency, for settling the problems in the known technology.